1. Field of the Invention
The present invention relates to a new process for the preparation of aqueous dispersions of polyurethanes containing chemically fixed carboxylate and/or sulphonate groups by the reaction of prepolymers containing free isocyanate groups with organic diaminocarboxylates and/or organic diaminosulphonates and/or organic dihydroxycarboxylates and/or organic dihydroxysulphonates in the aqueous or organic phase using isocyanate prepolymers which are virtually free from free monomeric diisocyanates.
2. Description of the Prior Art
Numerous processes are already known for the preparation of aqueous dispersions of ionically modified polyurethanes (see e.g. D. Dieterich et al, Angew. Chem. 82, (1970), pages 53 et seq or D. Dieterich, Die Angew. Makromol. Chem. 98 (1981), pages 133 et seq and the publications cited in these writings). One particularly simple method of preparing such dispersions comprises reacting isocyanate prepolymers based on organic polyhydroxyl compounds and monomeric diisocyanates in the aqueous or aqueous/organic phase with diaminocarboxylates and/or diaminosulphonates and optionally removing any organic solvent present by distillation after the reaction (see e.g. German Auslegeschrift No. 1,495,847 or German Offenlegungsschrift No. 2,035,732). In these methods known in the art, the isocyanate prepolymers used are reaction products of organic polyhydroxyl compounds with excess quantities of monomeric diisocyanates. These reaction products invariably contain a considerable quantity of free, unreacted monomeric diisocyanate even when only a small diisocyanate excess is used, so that when the prepolymers are reacted with the diaminocarboxylates or sulphonates, the formation of high molecular weight ionically modified polyurethanes is accompanied by the formation of polyelectrolytes with a high concentration of ionic centers formed as by-products due to the reaction of the monomeric diisocyanates with ionic chain lenthening agents. Polyelectrolytes generally lead to an increase of the particle size of the dispersed polyurethanes which increase is generally considered disadvantageous.
It has now been found that the properties of the aqueous polyurethane dispersions, in particular the mechanical properties of the sheet products obtained from them, can be substantially improved if the proportion of such polyelectrolytes in the dispersions is kept as low as possible. A further advantage of a low content of polyelectrolytes is to be seen in the fact that dispersions having a relatively high content of solids may be prepared. The low content of polyelectrolytes can be adchieved by freeing the isocyanate prepolymers used as starting components as completely as possible from their monomeric diisocyanate content before they are put into the reaction.